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Physics : Recent Questions and Answers (Page 18)

Find answers to your questions about Physics or help others by answering their Physics questions.

What does mutual induction of electric and magnetic fields in an EM wave mean?

Changes in the electric field makes changes in magnetic field and vice versa. What does this logically mean? It looks like an infinite loop. I don't understand how this helps...

Asked on 12/04/2021

2 answer

Does the absorbtion of photons from the Sun decrease with distance for a point charge?

Let's suppose we have a point charge with no dimensions (zero volume and surface area) absorbing photons from Sun. Does it change the amount of photons and energy absorbed when...

Asked on 12/04/2021

4 answer

Is conservation of charge valid in QM?

In electromagnetism, we have the continuity equation $$frac{partial}{partial t} rho(vec{r},t) = - vec{nabla} cdot vec{j}$$ which, upon integration over a finite volume $V$ bound by surface $S$...

Asked on 12/04/2021

3 answer

Gradient of function in terms of retarded time produces time derivative

In Griffith's introduction to electromagnetism on page 479, there is this following equation: begin{align*}nabla V &cong nabla left[frac{1}{4pi epsilon_0} frac{boldsymbol{hat r} cdot dot{mathbf{p}}(t_0)}{rc} right] \&cong left[frac{1}{4pi...

Asked on 12/04/2021

1 answer

Continuously feeding an evaporating micro-black hole?

What would happen if you created a micro-black hole and could continuously feed it as quickly as it evaporates? Is it possible that it would remain relatively stable? If...

Asked on 12/03/2021

2 answer

I am confused about the idea that the speed of light is independent of the speed of the source of light

Imagine that a mast of a sailboat is $186,000$ miles long, and the sailboat is moving to the right at a constant $10$ miles per second. If you...

Asked on 12/03/2021

4 answer

Proving that the angular velocity vector is equal to a limit involving the rotation vector

The angular velocity vector of a rigid body is defined as $vec{omega}=frac{vec{r}timesvec{v}}{|vec{r}|^2}$. But I'd like to show that that's equivalent to how most people intuitively think...

Asked on 12/03/2021

8 answer

How to prove the equivalence of two different definitions of $S$-operator?

I read there are two definitions about $S$-operator: The first one (e.g (8.49) in Greiner's Field Quantization) is:$$S_{fi}equiv langle Psi_p^{-}| Psi_k^{+}rangle$$where $|Psi_p^{-}rangle$...

Asked on 12/03/2021

2 answer

Is angular momentum conserved in case of a vehicle turning?

When a vehicle takes a turn is the sharpness viz radius of turn related to the velocity by conservation of angular momentum? If it is so then while going on...

Asked on 12/03/2021

4 answer

A problem related to metric variation

Under the coordinate transformation $bar x=x+varepsilon$, the variation of the metric $g^{munu}$ is:$$delta g^{munu}(x)=bar g^{munu}(x)-g^{munu}(x)=-frac{partial{ g^{munu}}}{partial x^{alpha}}varepsilon^{alpha}+ g^{mubeta}frac{partial varepsilon^{nu}}{partial x^{beta}}+g^{alphanu}frac{partial varepsilon^{mu}}{partial x^{alpha}}$$the right...

Asked on 12/03/2021

1 answer

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